Typically, a High Frequency Data Link (HFDL) protocol for airborne high frequency (HF) communications uses a single frequency for a plurality of aircraft to communicate with a ground station, and the ground station uses the same single frequency to communicate with the plurality of aircraft. The HFDL protocol is a time division multiple access (TDMA) protocol having a frame rate of 32 seconds for a single, one way communication transaction (e.g., aircraft transmits and ground station receives or ground station transmits and aircraft receives). A subsequent frame (e.g., a next frame 32 seconds later or a frame beyond the next frame 64 seconds later) includes a response or acknowledgment that indicates whether the communication transaction was successively received by the ground station. If the communication transaction was not successively received by the ground station, the aircraft waits and continuously tries again (e.g., up to another 64 seconds) and again (e.g., up to another 64 seconds), until the aircraft receives the acknowledgement from the ground station.
Since a plurality of aircraft can concurrently (or simultaneously) vie for 3-6 downlink slots available for each 32 second frame on the same single frequency, and the ground station transmits acknowledgements during the same 32 second frames on the same single frequency, there may be many transmission collisions resulting in many transmission fails and retries. The delay caused by transmission fails and retries is becoming increasingly unacceptable, and may not support new communication requirements from customers and certification bodies. Thus, an efficient and effective way to support multiple concurrent HF communication transactions may be desired.